1. Technical Field
The present disclosure relates in general to the field of water and wastewater treatment.
2. Background Art
A cloth disk filter typically comprises a tank or vessel, which may be concrete, metal, fiberglass-reinforced plastic (FRP) or other material. A source of screened, secondary effluent flows into the unit from an influent trough, and filtered effluent leaves the unit through an effluent trough. Influent may traverse an influent weir, a scum or hydraulic optimization baffle, then approaches one or more filter cassettes. In currently known systems, each filter cassette may have at least one dedicated backwash vacuum shoe assembly positioned on one side of filter cloth fluidly attached to the central rotatable backwash conduit. In some other known embodiments the filter cassettes rotate during backwashing, while the backwash shoes remain stationary.
Commonly used vacuum backwash shoes consist of using either a flat shoe having one or more suction orifices, or a shoe with lifts on the ends but still having a flat underside with a single slot/opening to sweep a single disk filter cloth clean. The limitations of these common approaches are the following. The shoes are only capable of cleaning a single cloth surface. The leading edge of the flat underside may compress, squeeze, and dislodge captured solids out of the reach of the suction of the backwash system; and even worse may force solids through the cloth to the filtered water side, thereby contaminating the filtered water. The flat underside may not provide for a volume of high turbulence that would enhance the cleaning/scouring effect by filtered water, and may not prevent “short-circuiting” of unfiltered secondary effluent into the shoe. Backwash shoes that raise the shoe off the cloth surface inherently weaken the impact of the suction cleaning Non-metallic shoes, while perhaps less expensive to fabricate compared with stainless steel shoes, represent a potential wear item over years of service. The sharp edges of these shoes naturally tend to abrade, fray, and otherwise wear the cloth surface.
The frequency of employing filtration with chemical treatment is increasing and it has achieved an important role in wastewater treatment. Particularly, chemical treatment is used in phosphorous removal, algae removal, metal hydroxides removal, and high solids removal applications. In those applications, usually the cloth filter disk shows very short filter run and frequent backwash. A reject rate of 20 percent is not uncommon.
There is a need in the cloth filter disk art for improved backwash mechanisms, and cloth disk filters and methods employing the improved backwash mechanisms in water and wastewater treatment facilities.